Brazilian President Jair Bolsonaro has authorised the use of the military to battle massive fires sweeping across parts of the Amazon as US President Donald Trump also offered his assistance.

Key points:

US President Donald Trump has offered assistance to Brazil as massive blazes continue to rage through the Amazon

Brazilian President Jair Bolsonaro has authorised the use of the military to help battle the fires

French President Emmanuel Macron questioned Brazil's trustworthiness and threatened to block a trade deal

Brazilian forces will deploy starting on Saturday to border areas, indigenous territories and other affected regions in the Amazon to assist in putting out fires for a month, according to a presidential decree.

The armed forces will collaborate with public security and environmental protection agencies, the decree says. Mr Bolsonaro's office confirmed that he had signed it.

The decree comes as Mr Trump tweeted out that he stood ready to assist if needed.

"Just spoke with President @JairBolsonaro of Brazil. Our future Trade prospects are very exciting and our relationship is strong, perhaps stronger than ever before. I told him if the United States can help with the Amazon Rainforest fires, we stand ready to assist!" Trump said in a post on Twitter.

Mr Bolsonaro has previously described rainforest protections as an obstacle to economic development, sparring with critics who note that the Amazon produces vast amounts of oxygen and is considered crucial in efforts to contain global warming.

Charred trees and fallen branches were seen around Porto Velho, the capital of Rondonia state, which borders Bolivia.

In some instances, the burned fields were adjacent to intact livestock ranches and other farms, suggesting the fires had been managed as part of a land-clearing policy.

A large column of smoke billowed from one fire, and smoke rose from a couple of nearby wooded areas.

France, Ireland threaten to block trade deal over fires

In escalating tension over the fires, France accused Mr Bolsonaro of having lied to French leader Emmanuel Macron and threatened to block a European Union trade deal with several South American states, including Brazil. Ireland joined in the threat.

The spectrum of possible economic repercussions for Brazil and its South American neighbours show how the Amazon is becoming a battleground between Mr Bolsonaro and Western governments alarmed that vast swathes of the region are going up in smoke on his watch.

Ahead of a Group of Seven summit in France this weekend, Mr Macron's office questioned Mr Bolsonaro's trustworthiness.

Brazilian statements and decisions indicate Mr Bolsonaro "has decided to not respect his commitments on the climate, nor to involve himself on the issue of biodiversity," Mr Macron's office said.

It added that France now opposes an EU trade deal "in its current state" with the Mercosur bloc of South American nations that includes Brazil, Argentina, Paraguay and Uruguay.

German Chancellor Angela Merkel views the fires as "shocking and threatening," according to government spokesman Steffen Seibert.

Argentina, which is struggling with rising poverty and austerity measures, has offered to send emergency workers to Brazil and Bolivia to help put out the fires. Chile has also offered aid.

The Brazilian Government has said European countries are exaggerating the country's environmental problems in order to disrupt its commercial interests.

Mr Bolsonaro, who has said he wants to convert land for cattle pastures and soybean farms, said it was difficult to curb increasing deforestation with limited resources.

"It's not easy to fight deforestation, our Amazon area is bigger than all of Europe," he said.